Patent GB-949573-A (GB)
Title: Process and apparatus for measuring and controlling the dust content of gases
IPC: G01N21/72
Filed: 1960-08-31
Abstract: 949, 573. Photo-electric control of dust extraction. METALLGESELLSCHAFT A. G. Aug. 31,1960 [Sept. 1959; Dec. 23,1959], No.30071/60. Heading G1A. [Also in Division G3] The dust content in a stream of gas is automatically controlled by passing the gas through a plurality of electrostatic precipitators 31a-31d, Fig. 5, connected in parallel, separately measuring the dust content at the output of each precipitator by photo-electric means and controlling the throughput of each precipitator by an associated input throttle member 42 controlled in accordance with the difference between the measured output dust content and a datum value of dust content which is dependent on the pressure difference between the input and output of the precipitator such that if the pressure difference exceeds a predetermined value the datam value of dust content is increased. As shown in Fig. 5 the gas from an input duct 30 is fed to the precipitators 31a-31d through respective input pipes 30a-30d, the output from respective output pipes 32a-32d are combined in a main output duct 32 and the pressure difference for each precipitator, e.g. precipitator 31a, is determined as the differential pressure between points 39a and 39h in the respective input and output pipes. When the gas is combustible, the dust content in each output pipe, e.g. 32a, is measured by a flame photometric method by coupling the output pipe to a burner 34a via a pressure reducing valve and drying means 35a, the output from an associated photo-electric detector 36a responsive to the colour or intensity of the burner flame being applied to a respective control system in a unit 38 to give a control signal which is applied to a relay 40 to actuate a motor 41 or magnetic device controlling the throttle 42. The control system may include a time delay, e.g. a damped pneumatic amplifier, to smooth out sudden variations. It is stated that in such a system, although there is no direct electrical interconnection between the control circuits, the effects of the control signals are collated by virtue of the interconnections of the inputs and outputs of the precipitators and that the total gas flow is substantially constant. Each photo-electric detector 36 may comprise a photo-electric element 12, Fig. 7, (e.g. cadmium sulphide photo-resistor, lead sulphide photoconductor or caesium cell) in a bridge circuit including a meter 29 and possibly a recorder; further details of the burner and detector are illustrated in Figs. and 2 (not shown). Each control system in the unit 38, Fig. 5, may be as shown in Fig. 7 in which an A.C. voltage proportional to the reading of the meter 29 is produced across a resistor 44 by a synchro 37, an A.C. voltage representing the datum value of dust content dependent on the pressure difference as measured by a differential pressure gauge 39 is produced across a resistor 43 by a synchro 47 and an A.C. voltage representing the position of the input throttle 42 is produced across a resistor 48 by a synchro 49, the three voltages being combined vectorially in a transformer 45 to give the control signal actuating the motor 41 controlling the throttle 42. If the gas is not combustible the sample fed to the burners 34 may be mixed with a combustible gas. Alternatively the dust content may be determined by a known turbidometer arrangement comprising a light source and selenium cell.